[app.simplenote.com] Stored XSS via Markdown SVG filter bypass

Hi, A carefully crafted injection used against the Markdown input parser can be leveraged to store and execute arbitrary JavaScript in the `app.simplenote.com` context. ## Proof of concept Before proceeding to reproduce this vulnerability, please log in to `app.simplenote.com` and create a new note with the "Markdown Formatted" option enabled. 1. Please paste the below payload into the "Edit" window, then select the "triple dots" icon > **Publish** 2. Next, please access the provided Simplenote URL, and select the black rectangle to execute the XSS payload. Please note that I deleted the note and account used to test the aforementioned PoC immediately after confirming successful exploitation. ### Markdown parser payload ``` <div id="137"><svg> <a xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="?"> <circle r="400"></circle> <animate attributeName="xlink:href" begin="0" from="blocked:alert(document.domain)" to="&" /> </a>//["'`-->]]>]</div> ``` ### Supporting evidence {F223223} ## Verified conditions At the time of testing, I have successfully confirmed exploitability in the following environment: * Firefox 55.0.3 stable (32-bit) on Ubuntu 16.04.3 LTS Thanks, Yasin